1. Field
The described technology relates to a system for displaying navigation aid information of an aircraft.
More particularly, the described technology relates to such a system for displaying navigation aid information of an aircraft, of the type comprising a display screen including at least one first display area further allowing the display of:                a first altitude scale including graduations;        the current altitude of the aircraft on the first scale;        a set altitude value.        
The described technology also relates to a primary flight display of an aircraft.
2. Description of the Related Technology
The field of piloting and more particularly tracking of the altitude of an aircraft has multiple links with characteristic altitudes.
Among these characteristic altitudes, mention may for example be made of the altitude selected by the flight plan, the altitudes for reducing thrust, the altitude constraints on the flight plan and other altitudes.
During a flight, the vertical situation of the aircraft is generally monitored by the pilot. In addition to the current altitude of the aircraft, this vertical situation is relative to other diverse elements.
These elements for example comprise the flight plan inserted into the flight management system (FMS) and the vertical profile calculated by the system. The vertical profile, for example, takes into account altitude constraints on certain points. These constraints should be observed.
The elements of the vertical situation may also comprise the set altitude value, transmitted by air control. This set value takes precedence over all the other constraints which may have been integrated into the flight plan.
The elements of the vertical situation may also comprise information on the overflown ground. This element is not fundamental since the procedures followed during the flight and the instructions from air control are assumed to ensure separation of the aircraft and from the ground. However, for monitoring purposes (in particular in the event when an error or a fault occurs), the situation of the aircraft relative to the ground is certainly of interest.
Finally, the elements of the vertical situation may comprise the capabilities of ascent or descent of the aircraft. The available thrust of the engines sets the ascent capability. The requirement for maintaining a particular speed sets the capability of descent. Thus, observance or non-observance of a forced altitude may be anticipated as regards the ascent or descent gradient which the aircraft may actually support.
Various systems for displaying one or several of these elements exist in the state of the art.
Thus, for example, the display system integrated into the primary flight display (PFD) of many aircraft includes at least one altitude scale able to display the current altitude of this aircraft.
Such a scale is displayed vertically on the primary flight display and includes uniformly distributed graduations along this scale.
Generally, the scale is centered around the current altitude of the aircraft with a visible portion on the screen extending up to 1,000 ft. above and 1,000 ft. below this current altitude.
This visible portion therefore corresponds to the extension of the scale around the current altitude.
The display system is also capable of displaying on such a scale, certain other elements of the vertical situation of the aircraft, such as for example the set altitude value.
Thus, the set altitude value for example corresponds to a graduation of this scale indicated with a suitable indicator.
However, this solution has a certain number of drawbacks.
In particular, because of the relatively small extension of the scale, such a display system does not allow the display of a set altitude value if the latter is too far from the current altitude.
Notably, in the example shown below, the extension of the scale corresponds to 1,000 ft. and if the absolute value of the difference between the current altitude and the set altitude value is greater than 1,000 ft., the display system no longer allows the display of the set altitude value on the scale.
In this situation, even if the numerical value of the set altitude value generally remains visible on the primary screen, the absence of the corresponding indication on the scale does not allow the pilot to become aware, at a glance, if this set altitude value is far away from or close to the current altitude.